1. Field of the Invention
The present invention relates to a method of mining a seam of rigid hydrocarbonaceous containing mineral such as oil shale or coal, utilizing a room and pillar mining technique whereby essentially complete recovery of the mineral deposit is possible. This method comprises the steps of extracting a portion of the mineral using a room and pillar mining technique and then filling the void areas which result with a structural grade concrete made up of the waste products from the mineral after the hydrocarbonaceous values have ben withdrawn. When the concrete hardens, it provides support for the overburden and allows the remaining mineral to be removed. Further, this invention discloses an effective way to dispose of the waste by-product after the hydrocarbonaceous values have been withdrawn and alleviates the need for surface disposal. Furthermore, this invention reduces or eliminates surface subsidence.
2. Description of the Prior Art
The most widely used method for mining underground deposits of minerals throughout the world is the room and pillar technique. This method has been utilized to a large extent in mineral mining for it has proven to be the most economical way to obtain the maximum extraction from the deposit while allowing compliance with the safety standards of the industry. By this method, the valuable minerals are mined in rooms separated by narrow ribs or pillars. This is accomplished by forming a series of tunnels or entries into the mineral seam and then extracting additional mineral material by working a series of rooms or tunnels off of the first entry way. There are many modifications of the basic method, such as: County of Durham system, double entry room and pillar mining system, double room system, double stall system, pillar and stall, room and stoop, single entry system, single stall system, etc. Generally speaking, there are two basic pillar systems--randomly spaced pillars and regularly spaced pillars or ribs. The selection of either system is based on the type of deposit that is to be mined and the depth of the overburden. For example, the irregularly spaced pillars are usually formed in deposits in which the areal mineralization is variable. In such a case, the rich areas are mined and the pillars are formed in the lean areas. The regularly spaced pillars, which are usually square or rectangular in shape, are used in deposits in which the mineralization is more uniform, such as in coal or other nonmetallic mineral mines. Perhaps the most commonly seen form of room and pillar mining is the rib pillar design system. In this particular design, the rib pillars are usually rectangular in shape and formed by the excavation of parallel openings in which the dimension parallel to the openings is much greater than the distance between the openings. The room and pillar technique has received great acclaim for it uses the sidewalls and unexcavated pillars to carry the high stress loads imposed by the overlying mineral and surface rock. Even though one uses a room and pillar mining technique, one must still leave a considerable portion of the mineral in place in the form of walls and pillars in order to support the overburden. It is known, for example, that in coal mining the recovery rate is seldom over 85%, and on the average the recovery rate for United States coal mines is only 50%. This factor is attributed to the stress factors present in every mine and the load distribution on each pillar. SME Mining Engineering Handbook, Volume 1, chapter 7, incorporated by reference and made a part hereof, describes the design and stability of excavations in subsurface formations. A volume-extraction ratio is disclosed on pages 7-41 and 7-42 and is defined as the ratio of the mined (or minerable) material to the total volume of the mineral deposit. This ratio will vary according to the compressive-stress concentration in the pillars and the particular mining arrangement employed.
It is obvious that the amount of recoverable mineral is dependent upon: (1) the stress load and (2) the arrangement and size of the pillars. Generally, rib pillars provide a very stable system for openings but for mineral mining they may not produce the best system for recovering the greatest amount of material. For example, to obtain a good extraction ratio, say 75 percent, the room width has to be three times the pillar width. For an effective pillar width that will produce a stable pillar, the room width may be so large that the roof is unstable. Usually, a better extraction ratio can be obtained with a three-dimensional system of random pillars, or with grids of regularly spaced square or rectangular pillars.
Another method that is used to increase the recovery rate in room and pillar mining is the operation in which a substantial portion of the pillars are removed upon retreating from the mine. This does, however, result in the partial or total subsidence of the land surface. Depending upon the location, it is sometimes impractical or impossible to permit subsidence for the miner may not own the surface rights or because disruption of surface drainage or tansportation systems may result.
In the case of oil shale, the mineral recovery rate may vary from as little as 15 percent to as high as 70 percent, depending upon the thickness of the oil shale seam and the depth of overburden. The following table gives an indication of typical recovery rates for room and pillar mining of oil shale as it relates to the depth of the mineral seam.
Table I ______________________________________ Depth of Cover - ft. % Left in Pillars ______________________________________ 200 50 200-500 50-60 500-1000 60-70 1000-2000 70-85 ______________________________________ "Oil Shales and Shale Oils", by H. S. Bell; D. Van Nostrand Co., Inc., Ne York; 1948.
For very thick seams of oil shale such as those of the Piceance basis of Colorado, which vary in thickness up to 1000 feet, the recovery rate by room and pillar mining may be even lower than those indicated in the above table. This low output is caused by the composition of the oil shale and the typical way a seam is worked. In oil shale, a horizontal working entry or horizon is usually no greater than 60 feet high. Since it is important to maintain the integrity of the roof and floor of the horizon, it is necessary to leave undisturbed a seam of 40 to 60 feet of virgin material between parallel working horizons. Accordingly, in the vertical plane, only alternate horizons of approximately 60 feet each, of a thick oil shale seam are mined. There arises also the necessity of columnizing the working horizons in order to adequately support the overburden.
Where practical, artificial pillars may be constructed to prevent or delay roof collapse prior to partial or total pillar removal. Artificial pillars can be constructed of wood posts, wood or concrete cribbing (which may or may not be filled with rubble or as taught in U.S. Pat. No. 1,004,419), or constructing a supporting crib of in situ rock blasted from the floor and ceiling of a mined-out coal vein and filling the voids with flushed mining waste. However, the strength and integrity of these artificial pillars is often unknown, making the removal of the mineral pillars hazardous. Also for very deep mineral seams the cost of constructing a sufficient number of artificial pillars for total roof support may well exceed the value of additional minerals recovered.
Further, it is known that the waste residue from hydrocarbonaceous containing minerals may be used in the production of a Portland type cement. Typical analyses of the residue from pyrolized coal and oil shale are shown in Table 2.
Table 2 ______________________________________ Weight Percent Mineral Matter Coal Ash Spent Shale ______________________________________ SiO.sub.2 35-50 30-35 Al.sub.2 O.sub.3 20-35 5-15 CaO 5-25 15-20 Fe.sub.2 O.sub.3 3-15 2-5 MgO 1-3 3-10 K.sub.2 O 0.5-2 1-3 SO.sub.3 2-7 1-3 ______________________________________
These analyses show primarily cement forming oxides that, when combined with a lime containing material and fired to a temperature of about 2800.degree. F., form a hydraulic cement. U.S. Pat. No. 3,759,730 describes a process for producing a Portland type cement from coal ash. The procedures for producing a hydraulic cement from spent shale are adequately described in U.S. Pat. No. 3,459,003. This latter patent further describes a method of pumping a slurry of cement made from spent shale into a body of spent shale particles previously deposited in slurry form into a mined-out area. The purpose of this procedure is to increase the density and therefore the amount of spent shale which may be disposed of in the mined-out area.
In mining the deposits of oil shale which exist in the United States, one can typically expect to recover approximately 10 to 50 gallons of oil per ton of oil shale. The extraction of the shale oil by pyrolysis produces a spent shale residue which is greater in volume than the original oil shale as mined. The spent residue therefore presents a serious disposal problem. It has been proposed that a substantial portion of this spent shale residue may be deposited in the mined-out area, thereby limiting the amount of residue which must be disposed of on the ground surface. Developed methods of depositing and compacting spent shale residues on the ground surface may, if not properly controlled, present serious environmental problems.
As to coal ash, the disposal of this waste residue presents a lesser problem for the typical coals available in the United States contain no greater than 15 percent ash. Although a similar disposal problem is present, the quantity of material to be disposed of greatly reduces the nature of the problem. It should also be noted that coal ash is produced at the location where the coal is burned not mined, and therfore the spent residue is not on site as it would be in the case of oil shale.